In particular, studies of nonauditory evoked potentials would benefit from simultaneous measurements, as these potentials can also be collected in the non‐silent (acquisition) periods and not only between MRI scans (the silent period). In addition, interleaved acquisition is often more time consuming, which is a problem in evoked potential studies that require averaging over many trials. The obvious disadvantage of this approach is that one loses potentially valuable EEG information during MR scan periods. This is why earlier combined EEG‐fMRI studies discarded the artifact‐contaminated segments and used only EEG ‐periods between two successive MRI scans (“interleaved” EEG‐fMRI). These artifacts, which occur during fMRI acquisition periods, have a wide frequency range and huge amplitudes of several millivolts, making the EEG (with amplitudes around 10–250 μV) unrecognizable. The second class of artifacts arising from RF pulses and gradients is much more problematic. Although a nuisance, this time‐ and frequency‐limited artifact leaves the EEG recognizable and utilizable for most purposes. It occurs with the heartbeat frequency (around 1 Hz). The amplitudes of this artifact are on the order of normal EEG amplitudes and the main frequency components lie between 4 and 7 Hz. The precise contribution of each component, however, is not yet fully known. ![]() It is generally thought that the cardioballistic artifact arises mainly from arterial pulse‐associated movements of the electrodes in the B0‐field and possibly to a lesser degree from electromotive force of blood ions. One artifact called the cardioballistic artifact depends on the high static magnetic (B0) field, another artifact is due to the switching of radiofrequency (RF) pulses and magnetic field gradients. The major obstacles in implementing simultaneous EEG‐fMRI, however, are the strong artifacts in the EEG signal that are induced by the magnetic field of the fMRI. Provided that the measured EEG signal measures net synaptic activity changes and not merely a change in synaptic synchronization and that the BOLD signal reflects synaptic activity changes as well, a combination of both methods could noninvasively localize synaptic activity changes in both space and time. Although EEG provides a high temporal resolution measure of synchronous synaptic activity, fMRI makes anatomical localization of neuronal activity associated blood oxygen level‐dependent (BOLD) signal possible at a millimeter‐level spatial resolution. Simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) is becoming increasingly attractive in neuroscience due to the complementary properties of the two techniques. Simultaneous continuous VEP‐fMRI recordings are thus shown to be feasible. This indicates sufficient artifact removal as well as physiological correspondence of VEPs in both periods. ![]() No significant differences between both VEPs were detected. ![]() Visual evoked potentials (VEPs) could be reconstructed reliably from periods during MR scanning and in between successive scans. In this study, continuous, simultaneous EEG‐fMRI measurements were carried out. ![]() An obvious disadvantage of this approach is the loss of a portion of the EEG information, which might be relevant for the specific scientific issue. Due to these artifacts, an interleaved modus has often been used for “evoked potential” experiments, i.e., only EEG signals recorded between MRI scan periods were assessed. Artifacts generated in the EEG signal during MR acquisition, however, continue to pose a major challenge. Simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) may allow functional imaging of the brain at high temporal and spatial resolution.
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